As the number of television channels available through various program delivery methods digital TV (DTV) broadcasting, cable TV, home video, broadcast, etc. continues to proliferate, the demand for programming, particularly high-quality HDTV-format programming, presents special challenges, both technical and financial, to program producers. While the price of professional editing and image manipulation equipment continues to increase, due to the high cost of research and development and other factors, general-purpose hardware, including personal computers, can produce remarkable effects at a cost well within the reach of non-professionals, even novices. As a result, the distinction between these two classifications of equipment has become less well defined. Although general-purpose PC-based equipment may never allow professional-style rendering of images at full resolution in real-time, each new generation of microprocessors enables progressively faster, higher-resolution applications. In addition, as the price of memory circuits and other data storage hardware continues to fall, the capacity of such devices has risen dramatically, thereby improving the prospects for enhancing PC-based image manipulation systems for such applications.
In terms of dedicated equipment, attention has traditionally focused on the development of two kinds of professional image-manipulation systems: those intended for the highest quality levels to support film effects, and those intended for television broadcast to provide “full 35 mm theatrical film quality,” within the realities and economics of present broadcasting systems. Conventional thinking holds that 35 mm theatrical film quality as projected in theaters is equivalent to 1200 or more lines of resolution, whereas camera negatives provide 2500 or more lines. As a result, image formats under consideration have been directed towards video systems having 2500 or more scan lines for high-level production, with hierarchies of production, HDTV broadcast, and NTSC and PAL compatible standards which are derived by down-converting these formats. Most proposals employ progressive scanning, although interlace is considered an acceptable alternative as part of an evolutionary process. Another important issue is adaptability to computer-graphics-compatible formats.
Current technology directions in computers and image processing should allow production equipment based upon fewer than 1200 scan lines, with picture expansions to create a hierarchy of upward-converted formats for theatrical projection, film effects, and film recording. In addition, general-purpose hardware enhancements should be capable of addressing the economic aspects of production, a subject not considered in detail by any of the available references.
For the first fifty years of television in the United States, the history shows continuous development and improvement of a purely analog-based system for video production broadcasting. The nature of the NTSC system is to limit the video bandwidth to 4.2 MHZ, which corresponds to approximately 340 TV-lines of resolution. In countries where PAL or SECAM systems are employed, the bandwidth is 5.5 MHZ, which corresponds to approximately 440 TV-lines of resolution.
During the past ten years, digital processing has become the standard for video production equipment. However, to preserve compatibility with existing equipment and standards, the video bandwidth typically has been limited to 4-6 MHZ (for NTSC and PAL applications, respectively). This also has tended to reduce the apparent generation loss during video production steps.
In the past five years or so, digital image compression technology has matured greatly. Furthermore, there are many incompatible standards, such as the different forms of JPEG systems, the-Quick-Time system, MPEG-1, and the numerous forms of the MPEG2 standard. In addition, the latest recording formats for video production have introduced a new set of variations, including the ¼-inch DVC-formats from Sony and Matsushita. While the signal deterioration characteristics of multi-generation analog-based production systems are well known, those imperfections resulting from diverse-format digital video compression and the conversions between these formats can be just as troublesome and unpredictable. In practice, these repeated steps of analog-to-digital (A/D) conversion and digital-to-analog (D/A) conversion, as well as data compression and decompression, introduce many signal artifacts and various forms of signal noise. Although digital video production promises multiple-step production processes free of generation losses, the reality is different, due to the repeated steps of A/D and D/A conversions, as well as data compression and decompression, present when utilizing the various incompatible image data compression formats.
Meanwhile, during the last twenty years, camera technology has advanced to a point far surpassing the performance of traditional production equipment. The video bandwidth capability has increased from 4.2 MHZ (corresponding to 340 TV lines of resolution) to approximately 12 MHZ (corresponding to nearly 1000 TV-lines of resolution). Because of the limitations of conventional broadcast and production equipment, most of the detail information produced by today's high-performance camera systems is lost.
For HDTV systems, one goal is to produce images having approximately 1000 TV-lines of resolution per picture height, which requires a bandwidth of approximately 30 MHZ. This, in turn, raises a new problem in terms of signal-to-noise ratio. While conventional broadcast cameras can produce signals having a S/N ratio of 65 dB, utilizing 10-bit digital processing, HDTV cameras typically produce signals having a S/N ratio of 54 dB, and utilize only 8-bit digital processing. In addition, the typical HDTV camera utilizes a 2 Megapixel CCD, in which the elements are approximately one-quarter the size of conventional broadcast cameras. This translates into a much lower sensitivity (a loss corresponding to 1-2 lens f-stops), higher levels of “smearing”, and lower highlight compression ratios.
Analog-based HDTV systems, such as the Japanese MUSE system, do not approach the design goal of 1000 TV-lines. In reality, only one quarter of the picture information is transmitted. Although the nominal reduced luminance bandwidth of 20 MHZ provides approximately 600 TV-lines of resolution per picture height in static program material, this resolution is drastically reduced to only 450 TV-lines where motion is occurring. The chrominance bandwidth is even further reduced by the sub-sampling scheme, to 280 TV-lines for the I-signal and 190 TV-lines for the Q-signal (in static scenes), and to 140 TV-lines for the I-signal and 50 TV-lines for the Q-signal (in moving scenes). Although this system provides a wide-screen aspect ratio of 16:9, it does not really qualify as a High-Definition Television System.
Because of the aforementioned compatibility issues, it is clear that conventional video recorders cannot match the technical performance of modern camera systems. Although “D-6format” digital recorders are available, the cost and complexity of such equipment place these units beyond the means of the vast majority of broadcast stations. Furthermore, the capability of conventional switchers and other production equipment still fail to match that of available camera systems.
Other recorders have been produced, such as the one-half-inch portable recorder (“Uni-HI”), but this system only achieves 42 dB signal-to-noise ratio, and records in the analog domain. These specifications render this unit unsuitable for multi-generation editing applications. Furthermore, the luminance bandwidth is only 20 MHZ, corresponding to approximately 600 TV-lines of resolution.
W-VHS (“Wideband-VHS”) recorders provide a wide aspect-ratio image, but only 300 TV-lines of resolution, which also renders this unit unsuitable for any professional applications. Other distribution formats (such as D-VHS) require the application of high compression ratios to limit the data-rate to be recorded, so these formats only achieve W-VHS quality (less than 400 TV-lines of resolution).
The newly-introduced HD Digital Betacam format (HDCAM) video recorder utilizes a 3:1:1 digital processing system rather than the 4:2:2 processing. However, it has a 24 MHZ luminance bandwidth corresponding to 700 TV-lines of resolution, and a narrower chrominance bandwidth. Although this system is clearly superior to any existing analog HDTV recording system, it still falls short of delivering the full resolution produced by an HDTV digital camera. Because of its proprietary image data compression format, the production process results in repeated data compression and decompression steps, as well as A/D and D/A conversions, which, in turn, results in many signal artifacts and various forms of signal noise.
In summary, the conventional technology for these markets utilizes professional cameras having a 30 MHZ bandwidth, and capable of 1000 TV-lines of resolution. However, they produce quality levels more characteristic of consumer-grade equipment (in terms of resolution and signal-to-noise ratio). In addition, the price of these systems is cost-prohibitive both on an absolute and also a cost/benefit basis, employing digital systems which produce only analog-type performance.